The current invention relates to the production of three dimensional structures on a substrate by Reactive-Ion Etching. It can be applied to materials such as semiconductor, glass, polyimide or any other which can be etched using a reactive ion plasma.
Three dimensional semiconductor structures are required for optical confinement (for example in visible/infrared lenses, emitters or detectors) and electromagnetic confinement (eg microwave inductors, detectors or sources).
A number of techniques are known for the fabrication of optical confining structures such as microlenses. For example, Hutley et. al. teach the formation of small discs of photoresist which, on heating to melt, are drawn into the shape of small lenses by surface tension. (Physics World, July 1991 pp27-32).
Liau et. al. teach the formation of a stepped structure by repeated applications of photolithography and bromine-methanol etching. Mass transport within this structure to form a lens shape is then effected, again by heating to melt (see Appl. Phys. Lett. 55 (2) 10 July 1989; The Lincoln Laboratory Journal, Volume 3, Number 3, 1990). Other methods of forming microlens arrays are detailed in xe2x80x9cMicro-optics has macro potentialxe2x80x9d Laser Focus World June, 1991. Methods which involve reactive ion etching typically involve repeated applications of photoresist and etch runs. This makes the fabrication process cumbersome.
According to this invention, a method of producing or modifying a three dimensional surface profile on a substrate comprises the steps of
(i) forming a mask of resist on the substrate such that some area of the substrate is protected by the mask and some area is exposed and
(ii) subjecting the substrate to a plurality of iterations
wherein each iteration comprises at least one resist etch and at least one substrate etch, the resist etch being carried out using a suitable resist etchant, which modifies the shape of the mask and hence the area of substrate exposed, and the substrate etch being carried out using a suitable substrate etchant from which the mask affords protection of the substrate, and which removes material from the areas of substrate which are exposed.
In a preferred embodiment, an optical concentrator is formed on the substrate.
In a preferred embodiment the substrate comprises a semiconductor material.
In a preferred embodiment the substrate comprises InSb.
In a further preferred embodiment the substrate etchant comprises a CH4/H2 plasma.
In a further preferred embodiment the resist etchant comprises an oxygen plasma.
In a further preferred embodiment, a Winston cone is formed in InSb heterostructure material.